The present invention relates generally to methods for making semiconductor transducers, and more particularly to the micromachining of a semiconductor resonator.
Sophisticated integrated circuit devices require internal timing circuits for proper operation and synchronized communication with other devices and systems. A common timing technique is the use of an oscillator circuit that responds to an external dedicated piezoelectric crystal, which is typically quartz. By cutting the quartz crystal to precise dimensions, it can be stimulated to oscillate at a precise natural resonant frequency. The resulting oscillations can be input to an oscillator circuit on board an integrated circuit chip. The IC chip can then perform its functions at a precise operating frequency. Though such quartz crystal resonators are very accurate, they are manufactured separately from the IC chip fabrication, and require assembly with the chip at a later manufacturing stage.
The present invention enables integration of a miniature resonator with an integrated circuit chip as part of the same semiconductor fabrication process. A beam of conductive material is formed over a cavity and supported at one end. The length and mass of the beam are accurately controlled during fabrication so that its natural resonant frequency can be maintained to a high tolerance. This natural frequency can be used to drive an on-chip oscillator circuit and associated timing circuits to permit accurate synchronous operation and external communication.
In its most preferred technique, a polysilicon beam is patterned to lie over laterally adjacent materials having highly differentiated etch rates. The fast-etching material is removed from beneath the beam using isotropic etching. The beam is left cantilevered over a cavity supported at one end by the slow-etching material. The beam is then trimmed using a laser to reduce its mass in a controlled manner, thereby precisely tuning the beam to a natural resonant frequency within a narrow predetermined range.